skip empty change addresses for makers

[adlai]

works towards #418 but does not enable this behavior maker-side; that would require recognizing when a subset of utxos match the cjout size precisely, and short-circuiting past the addition of a dust amount.

[chris-belcher]

I have read the diff and I think it's not wrong.

[chris-belcher]

Could someone else have a look at this before we merge, it contains modifications to verify_unsigned_tx() which is quite important for making sure all the coins go to where they should.

[AdamISZ]

Taking a look now, will feed back if I have a comment.

[chris-belcher]

We can surely merge this now?

[adlai]

This should be merged, as it addresses #27 which is required by #171

[coveralls]

Coverage increased (+0.05%) to 79.916% when pulling 226bcf84bf83b66ceebbb2aa7cc7e456ca8ad20d on adlai:sweep-empty-change into f3afd81dc84633acfc83de753879f49d31520066 on JoinMarket-Org:develop.

[coveralls]

Coverage increased (+0.003%) to 78.117% when pulling 855c8663d1205f595f1e8ec8f23ea0950dd59d51 on adlai:sweep-empty-change into 46fe3e8bbe9bbc769db1732029e90db9591fe8d5 on JoinMarket-Org:develop.